By logging into one's account. Surely you see the distinction between deliberately availing oneself of a service and bring tracked on entirely separate websites without being informed, much less consenting.
You're making an argument against tracking in general. That's fine, but it's not what we're discussing. What we're discussing is if sites should treat traffic from browsers in incognito mode differently than traffic from browsers not in incognito mode. Do you think they should? I would argue that sites shouldn't even know whether or not their users are in incognito mode.